The present invention relates to an interconnection for circuits, and more particularly, to an interconnection for high-frequency circuits.
Microcircuit electronic products are typically assembled from individual components assembled into packages. In practice, individual components are picked and placed on a substrate. Substrates may be flexible polymer films or rigid ceramic pieces. Substrates may have multiple layers of circuit traces with interconnections between layers made in the Z direction through vias. Some components can be directly electrically attached to pads on the substrate with solder or adhesive or the like. Other components have connection pads that do not lie in the plane of the surface of the substrate. To accommodate the height of these pads, a technique such as wire bonding is used to route signals between the pads that lie in different planes.
For example, it has been common practice to glue or solder integrated circuits to a ceramic substrate and then to make circuit interconnections with wire bonds between pads on the integrated circuit (IC) and a pad on the substrate. The wire bond is a small loop of wire that leaves the pad on the IC and loops over the edge of the IC and drops to the surface of the substrate. Wire bonds are rapidly stitched onto the IC during production and a typical throughput is 5 wire bonds per second. Both via and wire bonds form acceptable signal paths for the package as long as the signal of interest is low frequency.
However, wire bonds both attenuate and radiate signals beginning in the low gigahertz range. The wire itself has a parasitic inductance, which becomes a significant circuit element in the gigahertz range. The conventional solutions to reducing the attenuation have included both wedge bonding and ribbon bonding. Both of these techniques are efforts to reduce the length of the wire and therefore to reduce the inductance of the wire.
In the wedge technique, the wire is routed at an acute angle off the pads to minimize the total length of the connection. Such connections can be made at a rate of approximately 2-3 bonds per second. Ribbon bonds substitute a flat ribbon wire for the conventional wire used for wire bonding, which has a circular cross section. The ribbon has more parasitic capacitance and less parasitic inductance than a round conductor, which minimizes attenuation. However, successful ribbon wire bonding is critically dependent on component placement. This process is also extremely slow and thus the higher cost.
A similar signal routing problem arises on the substrate itself. In a multi-layered substrate, high frequency or high-speed signal traces typically lie close to a ground plane. As a result, signal paths can be designed with constant nominal impedance using well-known stripline or microstrip techniques for RF signals. However, if the signal path passes in the Z direction then the asymmetry may create an impedance mismatch with an associated attenuation and reflection. Ceramic substrates are particularly prone to this problem because they are brittle and additional thickness in the Z-axis is required for mechanical strength. The increased thickness and high dielectric constant exacerbates the impedance mismatch problems.
Therefore, there is a need for improved interconnection devices and methods for production of high-performance, high frequency circuitry packages.
One aspect of the present invention is directed to a method of interconnecting a first pad to a second pad. The method comprises providing a wire extending between a first pad and a second pad, the wire forming at least a portion of a signal path; and providing a capacitor, thereby tuning the portion of the signal path formed with the wire to an impedance that substantially matches an impedance of at least one other portion of the signal path at one or more predetermined frequencies.
Another aspect of the invention is an apparatus for connecting a first portion of an electrical circuit to a second portion of an electrical circuit. The apparatus comprises a wire providing electrical communication between the first and second portions of the electrical circuit. The wire forms at least a portion of a signal path. A capacitor is electrically proximate the wire. The capacitor provides the wire with a characteristic impedance over a design bandwidth, wherein the characteristic impedance substantially matches an impedance of at least one other portion of the signal path at one or more predetermined frequencies.